1. Field of the Invention
The present invention relates to an indicator for an optical instrument, for example, an indicator applied to a surveying instrument such as an auto-level or a transit instrument having a telephotographic system.
2. Description of the Related Art
A surveying instrument such as an auto-level or a transit instrument is basically provided with a collimating telescope, a level, and scales for measuring a rotative angle (an azimuth angle) or an elevational angle. A typical auto-level collimating telescope is provided, in order from an object side, with an objective lens group, a focusing lens group, a horizontal compensation and erecting optical system, and an eyepiece lens group. The position of the focusing lens is adjusted according to the distance from the object, so that an image of the object may be formed on a reticle (focusing plate). The operator may thus observe the image superimposed on the reticle via the eyepiece.
Surveying instruments, such as an auto-level, did not have a distance measuring apparatus that could indicate the distance to an object, for example to a staff. In particular, although the auto-level may preferably be located at the equal distance position from two measuring points, the auto-level has no function to measure the distance. Therefore in the prior art, the position of the auto-level has usually been decided according to experience and intuition of the operator. Accordingly, it would be convenient if the distance to the measuring point could be made known to the operator.
A collimating telescope of surveying instrument in which an automatic focusing apparatus is provided is well known. According to a conventional automatic focusing apparatus, even if accurate focusing is not carried out, as long as the defocus is little enough so that the operator may feel as though the image is focused, the focusing operation would be stopped. However, the measuring of the object distance with the existence of such a defocus may result in a large object distance error.